IT WAS 2 August 1999, a particularly hot day in Cirencester, Gloucestershire.
The Luker family had just finished lunch and were on their way into the Bishops
Walk shopping centre. 鈥淲e鈥檇 just walked in the main door when I heard this
tremendous bang,鈥� recalls Lynne Luker. 鈥淚 looked up and saw a huge shower of
glass falling towards us. In the centre was a big chunk, about the size of a
basketball, hurtling straight for my five-year-old son. By coincidence my
eldest, Ashley, had just leaned over to speak to him and the glass slammed onto
the back of his neck. A split second later the two lay in front of me covered in
产濒辞辞诲.鈥�
Fortunately, neither boy was killed, although Ashley鈥檚 knees were seriously
damaged when he fell. An investigation by Cotswold District Council found that a
large pane of toughened glass in the roof had shattered without warning and
fallen from its frame onto the Lukers. This was the third time a pane had failed
at Bishops Walk.
When fragments were analysed by experts at the giant glass manufacturer
Pilkington, which had made the pane, Ian Wheeler, the council鈥檚 environmental
health officer, was told that minute crystals of nickel sulphide trapped inside
the pane had almost certainly caused the failure. The Luker family had no idea
that these manufacturing impurities could shatter toughened glass without
warning. But for many engineers it鈥檚 an all too familiar problem.
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鈥淥n average I see about one or two buildings a month suffering from nickel
sulphide related failures,鈥� says Barry Josie, an engineer at building
consultancy Bickerdike Allen Partners, which was involved in the Bishops Walk
investigation. 鈥淭hat was a relatively minor outbreak.鈥�
Other experts tell of similar experiences. Tony Wilmott, of London-based
consulting engineers Sandberg, and Simon Armstrong at CladTech Associates in
Hampshire both say they know of hundreds of cases. 鈥淚n this country it happens
all the time,鈥� says Armstrong. And the Luker boys are not the only casualties.
鈥淧eople have been lacerated as a result of falling toughened glass,鈥� he
says.
The glass industry is aware of the issue, says Brian Waldron, chairman of the
standards committee at the Glass and Glazing Federation, a British trade
association, and standards development officer at Pilkington. But he insists
that cases are few and far between. 鈥淚t is a very rare phenomenon,鈥� he says.
Armstrong and others disagree. 鈥淲hat you hear is only the tip of the
iceberg,鈥� says Trevor Ford, a glass expert at Resolve Engineering in Brisbane,
Queensland. He believes the reason is simple: 鈥淣o one wants bad press.鈥�
Toughened glass is found everywhere from cars and bus shelters to the
windows, walls and roofs of thousands of buildings around the world. It鈥檚 easy
to see why. This glass has five times the strength of standard glass, and when
it does break, it shatters into tiny cubes rather than large, razor-sharp
shards. Architects love it because large panels can be bolted together to make
transparent walls, and turning it into ceilings and floors is almost as
easy.
It is made by heating a sheet of ordinary glass to about 620 掳C
to soften it slightly, allowing its structure to expand, and then cooling
it rapidly with jets of cold air
(see Diagram).
This causes the outer layer of the pane to
contract and solidify before the interior. When the interior finally solidifies
and shrinks, it exerts a pull on the outer layer that leaves it in a permanent
compression and produces a tensile force inside the glass. As cracks propagate
best in materials under tension, the compressive force on the surface must be
overcome before the pane will break, making it more resistant to cracking.
The problems start when glass contains nickel sulphide impurities. Trace
amounts of nickel and sulphur are often present in the raw materials used to
make glass, and nickel can be introduced by fragments of nickel alloys falling
into the molten glass. As the glass is heated, these atoms react to form tiny
crystals of nickel sulphide. Just a tenth of a gram of nickel in the furnace can
create up to 50,000 crystals.
These crystals can exist in two forms: a dense form called the alpha phase,
which is stable at high temperatures, and a less dense beta phase that is stable
at room temperature. The high temperatures used in the toughening process
convert all the crystals to the dense, compact alpha form. But the subsequent
cooling is so rapid that the crystals don鈥檛 have time to change back to the beta
phase. This leaves unstable alpha crystals in the glass, primed like a coiled
spring ready to revert back to the beta phase without warning.
When this happens, the crystals expand by up to 4 per cent. And if they are
within the central, tensile region of the pane, the stresses this unleashes can
shatter the whole sheet. The time to failure is unpredictable. It could happen
just months after manufacture, or decades later, although if the glass is
heated鈥攂y sunlight, for example鈥攖he process is speeded up.
Ironically, says Graham Dodd, of consulting engineers Arup in London, the oldest
pane of toughened glass known to have failed due to nickel sulphide inclusions
was in Pilkington鈥檚 glass research building in Lathom, Lancashire. The pane was
27 years old.
The first recorded case occurred in Australia in the late 1950s when windows
at ICI House in Melbourne broke suddenly. Since then, publicised cases have
included the Business Promotion Centre in Duisburg, Germany and Waterfront Place
in Brisbane, Australia. Britain has suffered particularly badly: buildings that
have been affected include the Eurostar rail terminal at Waterloo in London, the
Reuters Building in London鈥檚 Docklands and Stansted Airport鈥檚 passenger
terminal. Most have resulted in costly refurbishment and some in legal disputes.
And according to engineers, at least five other buildings in London, Hong Kong
and Sydney are currently experiencing failures due to nickel sulphide
inclusions.
Nothing to hide
鈥淢any architects were never made aware of the problem when they designed
buildings in the 80s and early 90s,鈥� says John Barry, an expert on nickel
sulphide contamination at the University of Queensland in Brisbane. Josie
believes that glass manufacturers are not being as open about the problem as
they could be. 鈥淭hey don鈥檛 want to admit they鈥檝e got it because of the potential
legal liability they might open themselves up to,鈥� he says. Willmott agrees, and
believes that this reticence has prolonged the problem. 鈥淭here鈥檚 been a
guardedness from the industry,鈥� he says.
Mark Brew, senior technologist at Glass Technology Services in Sheffield,
part of a trade association called the British Glass Manufacturers
Confederation, doesn鈥檛 agree. 鈥淭here are people in the industry who wish it
wasn鈥檛 there,鈥� he says, 鈥渂ut I don鈥檛 think we鈥檙e hiding it. There are articles
in the press.鈥� Besides, says Waldron, manufacturers have made huge strides in
eliminating the problem. 鈥淧roduction lines [at Pilkington] have achieved purity
levels of one nickel sulphide inclusion in 7 tonnes of glass: that鈥檚 a minute
proportion,鈥� he says. 鈥淭here鈥檚 more toughened glass broken through vandalism and
bad installation than through nickel sulphide.鈥�
No one disputes that vandalism causes more failures, and plain bad luck can
sometimes strike. One pane in the roof of the Eurostar terminal is thought to
have broken when a passing bird of prey dropped its dinner鈥攁 deer鈥檚
hoof鈥攐nto it.
But data showing the scale of the nickel sulphide problem are almost
impossible to find. Brew says he sees about a dozen cases a year. 鈥淚 don鈥檛 know
how many other people find.鈥� The picture is made more complicated by the fact
that these crystals occur in batches. So even if on average there is only one
inclusion in 7 tonnes of glass, if you experience one nickel sulphide failure in
your building, that probably means you鈥檝e got a problem in more than one pane,
says Josie. 鈥淚n the past decade I鈥檝e worked on over 15 buildings with the number
of failures into double figures,鈥� he says.
One of the worst examples of this is Waterfront Place which was completed in
1990. Over the following decade, the 40-storey Brisbane block suffered a rash of
failures. Eighty panes of its toughened glass shattered due to inclusions before
experts were called in, says Barry.
Barry and Ford analysed every pane of glass in the building. Using a studio
camera, a photographer went up in a cradle to take photos of every pane. These
were scanned under a modified microfiche reader for signs of nickel sulphide
crystals. 鈥淲e discovered at least another 120 panes with potentially dangerous
inclusions which were then replaced,鈥� says Barry. 鈥淚t was a very expensive and
time-consuming process that took around 6 months to complete.鈥� Though the
project cost A$1.6 million (nearly 拢700,000), the
alternative鈥攔e-cladding the entire building鈥攚ould have cost ten
times as much.
Ford has developed a new inspection system to examine windows in a building
in Hong Kong. His apparatus consists of a gantry housing a laser that scans the
glass. By analysing the scattered light, the system returns the coordinates of
any inclusions found. Engineers inspect the locations with a powerful lens to
determine if the inclusions are nickel sulphide crystals. 鈥淭he system scans 3
square metres per hour,鈥� says Ford. 鈥淏ut I believe that can be sped up
significantly by, for example, using multiple laser arrays.鈥�
Scanning a whole building for nickel sulphide is slow and expensive, but
glass manufacturers haven鈥檛 yet found a way to eliminate these inclusions at
source. However, they do try to prevent those that find their way into the glass
from doing any damage.
In the 1960s, Pilkington and French glass manufacturer Saint-Gobain developed
a process called heat soaking. This artificially ages toughened glass by heating
it for several hours. The idea is that this converts most nickel sulphide
crystals into the beta phase, causing any contaminated panes to fail in the
furnace. The panes that survive should be virtually guaranteed to be
inclusion-free.
However, to date, there is no universally accepted standard for heat soaking.
The only one in existence鈥攖he German DIN standard, introduced in the late
1980s鈥攔ecommends heat soaking for 8 hours at a temperature of 290 掳C.
But even in Germany it is only mandatory for some kinds of toughened glass. And
heat soaking can make the material two to five times as expensive. To keep costs
down, some companies heat soak for a shorter time or at a lower temperature.
鈥淭here is still a big debate on how long heat soaking should be carried out for
and to what temperature the glass should be heated,鈥� says Dodd.
A European standard is now being drafted, based on work by Andreas Kasper at
Saint-Gobain鈥檚 research centre in Germany that was published last year in Glass
Science and Technology (vol 73, p 130). It claims to turn out panes of toughened
glass with no more than one nickel sulphide inclusion in every 400 tonnes. The
standard is expected to receive official endorsement within two years.
But Mike Brungs, a glass expert at the University of New South Wales in
Sydney, believes such a move would be premature. 鈥淜asper used pure nickel
sulphide crystals, without impurities, in his experiments,鈥� says Brungs. This is
very different from the conditions in most manufacturers鈥� glass, he suggests.
鈥淢ost nickel sulphide crystals contain some form of impurity such as iron.鈥�
Because iron drastically slows the conversion time of nickel sulphide crystals
during the heat soaking, this could render the new European standard inadequate,
says Brungs. 鈥淚n practice I think the heat soaking times will need to be much
濒辞苍驳别谤.鈥�
Even if they are extended, there鈥檚 no guarantee that all the crystals will be
converted. 鈥淗eat soaking is never foolproof,鈥� says Josie. 鈥淎t least six of the
13 panels that failed in the 10,000 square metres of heat-soaked, toughened
glass in the Eurostar terminal were due to nickel sulphide inclusions.鈥�
And, says Willmott, even if the proposed standard achieves its aim, that
won鈥檛 rule out failures entirely. Large buildings can easily use 400 tonnes of
glass, which means there鈥檚 a good chance there will be one nickel sulphide
failure in it, he says.
As the Luker family discovered, toughened glass poses the biggest problem
when installed overhead. If it is not perfectly vertical, a broken pane is
likely to fall out of its frame onto whatever or whoever is below. And although
toughened glass breaks up into small pieces, sometimes they can hold together
forming large and dangerous lumps.
Britain has seen several cases in which people were injured by falling
toughened glass. A glass screen above an entrance to The Glades shopping centre
in Bromley, Kent, shattered in January 1993. Six people were hit and four had to
be treated in hospital. One shopper was injured by glass at the Harlequin
shopping centre in Watford, and a packed restaurant was sprayed with glass at
the Lakeside mall in Thurrock, Essex. According to Paul Stickney, chairman of a
technical affairs committee set up by the British Council of Shopping Centres to
investigate these failures, some of them were due to nickel sulphide. 鈥淚 don鈥檛
know how many鈥攁 couple of dozen panes maybe,鈥� he says. 鈥淏ut no one was
badly injured: a few cuts and scrapes at most.鈥�
Most buildings where this kind of breakage has occurred, including Bishops
Walk and the Eurostar terminal, have now either been reglazed or had safety mesh
fitted underneath the panes. Stephen Ledbetter at the Centre for Window and
Cladding Technology at the University of Bath has another solution. 鈥淚n
high-risk situations, where people might be below, the glazing unit should be
made up from a pane of toughened glass on the upper surface and a laminated pane
on the underside. If the toughened glass were to fail and fall out of its frame
it would be caught by the laminated glass layer beneath it.鈥�
Laminated glass鈥攖wo sheets of glass stuck together with
adhesive鈥攊s more likely to stay in its frame when it fails. Most large
cladding companies now recommend laminated glass in overhead glazing, says
Willmott. But it is pricey stuff, and he warns: 鈥淭here are an awful lot of small
companies who aren鈥檛 aware of the risks.鈥�
In nearly all European countries regulations introduced in the 1980s
effectively specify that overhead glazing must be laminated. But Britain is an
exception. 鈥淚t鈥檚 apparent that people haven鈥檛 learned their lessons,鈥� says
Armstrong.